Bolometer



March 4, 1952 Q B, AlKEN 2,587,674

BOLOMETER Filed April 15. 1950 IN VEN TOR.

4 I-ICHLES B. lKEN 49M 1.- BYL/J, f?

v IE' Arm/293) M HGENT Patented Mar. 4, 1952 BOLOMETER Charles B. Aiken, Wilton, Conn., assignor to the United States of America as represented by the 'Secretary ofthe Air Force Application April y13, 1950, Serial N o. 155,731

(Cl. 20L-63) 4' Claims.

The present invention relates to thermal responsive devices and more particularly to new and'improved bolometers for detaching radiant energy in the infra-red region, the intensity of which varies periodically at a relatively low second passage through the metal nlm and rate, although they are not limited to such use. blackening material a larger portion is absorbed The bolometers used, heretofore, for detectand the einciency is thereby raised. Moreover, ing radiant energy have comprised, generally, it providesv a conductive cooling eiiect whichis a narrow, thin strip of metal deposited on a substantially independent of frequency, up to a iilm of insulating material and enclosed in an Certain point, so that a better frequency charenvelope containing hydrogen at a very low presacteristic results than could be obtained with sure. Bolometers of this type, when carefully air at atmospheric pressure and no cooling plate. made, have good sensitivity and low-frequency It also tends to reduce mass motion of the gas response. It has beenl found, however, that immediately surrounding the metal strip,'thus leakage of minute quantities'of air into the en- 15 reducing IniCrODhOnC disturbances that might velope, or outgassing from the elements ccnotherwise be produced.

tained therein, can produce a marked reduction The invention may be better undelStOOd from in the cooling effect of the hydrogen, resulting the following detailed description of several repin an increased sensitivity and bad distortion of resentative embodiments, taken in conjunction the frequency:characteristic. The reduction in with the accompanying drawings in Which the cooling effect of the hydrogen may in some Fig. l is a View in longitudinal section illustratcases be suicient to cause they bolometer eleing a bOlOmeteI COnStIllCted aCCOldiner t0 the ments to overheat and burn out even at normal present inVentiOn;

operating currents. Fig. 2 is a view in transverse section taken The principal object 'of the invention, accord- 25 aliene line 2--2 0f Fig. l, looking in the direction ingly, is to provide bolometers responsive to in- 0f the erlOWS and ShOWing the metal and plastic fra-red radiant energy of varying intensity that lInS tOeethel With the eleCtIOde Structure; are free from the above-noted deiiciencies of FieA 3 iS e detailed VieW Showing a metal C001- prior art devices, and which are simplel rugged ing plate used in the bolometer illustrated in and easy to construct. Fig. l;

Another object of the invention is to provide Fie. 4 is a detailed View Showing the structure new and improved bolometers of the above char- 0f the eleCtrOdeS emplyed in the b010meter i1- a'cter in which the envelope contains air or an lustreted in Fgl; inert gas instead of hydrogen and 'in 'which the Fig. 5 is a view in longitudinal section showing gas pressure is appreciablygreater than the presthe details of a modified form of bolometer consure generally used in hydrogen bolometers. structed according to the invention;

A further object of the invention is to provide Fig. 6 is a View in transverse section taken new and improved bolometers of the above charalong line 6 6 of Fig. 5, looking in the direction acter in which the cooling effect' of the strucofthe arrows; tural elements is increased'relatively to the cool- 40 Fig. 7 is a detailed view of a cooling plate asing eiiect of the gas, whereby dependence of the sembly used in the bolometer of Fig. 5; frequency response on the cooling effect of the Fig. 8 is a detailed view showing the electrode gas may be reduced. structure of the bolometer in Fig. 5;

The objects of the invention are attained in a Fig. 9 is a partial View in longitudinal section b'ol'ometer having a blackened metal4 film de- 45 illustrating another embobiment of the invenposited on an' insulating filmv by disposing a tion; polished cooling plate in heat conducting rela- Fig. 10 is a top view of the embodiment shown tionto the metal and insulating' iilms; andby in Fig. 9; and introducing `into the bolometer envelope 'air' or f Fig, 11 is a Schematic diagram illustratingapan inert' 'ses 'at a suitable pressure. 'With-the 50 params for forming the metal strip of a bolomenl'ielepeeinebetween the'c'Ooling plate and the ter in accordance with the invention. metal and heat insulating illrns and with a suit- Referring now to Fig. 1, the bolometer com.. able gas pressure a bolometer having good senprises a lower cylindrical base member I2 which sitivity and low-frequency response can be ob- 'his open at the bottom and which has an upper tained. transverse partition I3 in which are formed a.

f The polished vcooling plate reflects mostof the energy passing through. the transparent insulating nlm and the blackened metal nlm deposited thereon which is so thin that it transmits a substantial fraction of the total received energy. By reflecting this transmitted energy back for a.

pair of spaced-'apart apertures I4 and l5. Se-

cured lwithinthe apertures It and l5, respec tively, are a pair of conventional insulating beads I6 and I1, respectively, carrying conducting supports I8 and I9, respectively. Mounted at the upper ends of the supports I8 and I9 are a pair of electrodes 20 and 2|, respectively, which may be shaped as shown in Fig. 4 to form a circular disk having a substantially rectangular aperture 2| in the center thereof, and which are separated by short gaps 23 and 24.

Formed on the electrodes 2B and 2| and covering the aperture 22 is a plastic iilm 25 on which formed a narrow metal strip 26, the upper surface of which is preferably blackened at 21 by evaporating gold thereon under a low pressure in the manner disclosed in the co-pending application of William I-I. Carter, Jr., filed March 23, 1946, Serial No. 656,640, now Patent No. 2,562,770, issued July 3l, 1951. The ends of the metal strip 26 are electrically connected to the conducting segments 28 and 29 (Fig. 2) which may be of gold deposited on the electrodes 2| and 20 as shown and in a manner to be explained later.

Secured beneath the electrodes 20 and 2| by a suitable insulating cement and spaced a short distance therefrom, is a cooling plate made of copper or tin-plated iron, for example, and

having a central, polished raised portion 52. The raised portion 52 should preferably have substantially the same dimensions as the metal strip 26 and its upper face should be located a predetermined, short distance below the metal strip 26, as described in greater detail hereinafter.

The base member I2 is mounted within a cylindrical envelope 30 which may be secured to the lower base member I2 at the lower end thereof In order to form the film 25 on the electrodes 20 and 2 I, the bolometer subassembly comprising the lower base member I2, the insulating beads I6 and I1, supporting rods I8 and I9, and'electrodes 20 and 2| may be immersed in a container of water such that the electrodes` 20 and 2| lie a short distance below the water surface. A small quantity, preferably only a few drops of the cellulose nitrate solution prepared as described above is deposited on the water surface in any convenient manner such as by means of a medicine dropper. In this fashion, a lm of cellulose nitrate approximately 0.8 to 0.14 microns thick is formed on the water surface. When this lm has hardened somewhat, the water is slowly siphoned out of the container in such fashion that the film settles on the electrodes 20 and 2|; the excess cellulose is cut away and the mounted films are then baked for a period of approximately two hours at a temperature in the range from 140 to 145 F.

The cellulosic material is then removed fromA can be deposited by evaporation. Masks arethen in any suitable manner as by a low temperature solder 3 I, for example, in such fashion as to form an air-tight joint. In the upper wall of the cylindrical envelope 3|] is formed an inwardly dished portion 32 in the bottom of which is formed an aperture 33 communicating with the interior of the bolometer. Fitted within the inwardly dished portion 32 is a window 34 made of a material that is transparent to infra-red radiation such as sodium chloride or silver chloride, for example. The window 34 may be tightly cemented in the inwardly dished portion 32 by a small quantity of fused material 35 which may be the same as the material comprising the window 34.

The plastic i'llm 25 may be made of any suitable cellulosic material than can be mounted on the electrodes 20 and 2| in the form of a thin lm. A film solution suitable for this purpose may be made by dissolving approximately 5 feet of Ansco supreme, 35 mmfpanchromatic, nitrate base lm in 350 cc. of a suitable solvent such as amyl acetate, for example. The sensitized emulsion is first removed in any suitable manner as by immersing the lm in boiling water, for example, until the sensitized emulsion has been completely dissolved. It is then washed thoroughly in water and dried with a cloth, following which it should hours, after which is is stirred in orderto insure complete solution of the film. The i-llm solution is then iiltered in any suitable manner, as through a clean, cotton cloth, for example, to remove any gelatin and dirt that may be present and amyl acetate is added to replace any loss throughevaporation and to bring theyolume-of `the sclu;

tion up to 350 cc.`

tted over the electrodes 20 and 2| so that portions corresponding to the areas 28 and :29 in Fig. l2 are exposed while the rest of the surface is covered. A suitable metal, preferably gold, is then evaporated on the exposed areas 28 and 29 as described in greater detail below.

The evaporation of gold on the electrodes may be accomplished by means of the apparatus shown in Fig. ll. In Fig. ll is shown a vacuum chamber 36 such as a bell jar, for example,from which air is evacuated through conduit 31 by means of a suitable pump 33. The conduit 31 has a branch 39 communicating with a suitable pressure gauge 40 by means of which the pressure within the bell jar 36 may be measured. The gold to beused in the evaporation process is fused in a series of droplets 4| on a tungsten wire or iilament 42 which is connected at its opposite ends to the posts 43 and 44 formed of brassor other suitable conducting material.

The posts 43 and 44 are mounted in insulators 45 and 46 sealed in the base 41 of the bell jar 36 and are connected with a suitable source of electrical energy. The source of electrical energy may be a battery or, as illustrated, an alternating current generator 48 which is connected through a transformer 49 to the posts 43 and 44. A potentiometer or a variable ratio transformer 50 is connected between the generator and the transformer 49 in order to regulate `the current supply in the iilament 42.

The electrode assembly with the plastic nim 25 formed thereon is placed within the bell jar 36,. which is then evacuated to a pressure in vthe range of 10-5 to 10-6 mm. of mercury and electric current is passed through the filament 42 to evaporate gold from the droplets 4| 0n to the unmasked areas of the electrodes 2D and 2|. lA

satisfactory deposit can be obtained'in about 30. seconds by passing 13 amperes through a .020

diameter tungsten filament 425 having 7 droplets of gold thereon, the total weight being approximately l dwt.

The electrode assembly is then removed from the bell jar 36 and the electrode masks areremoved and replaced with other masks shaped to leave a central str1'm A' .bavingtbewme .width-estrie-. desired width of the metal strip 26 andV suiciently long/to roverlapl the' electrode 'golddepo'sit The electrode assembly is then replaced iin. lthe bellfiar 36 andthe xpressure isragain V'reduced-Tte thevrange' frcmf1-0-5 to 1'06-.mm."of mercury, whereupon goldA is .evaporated from the lament 42f-to` form thel metal strip 26. .If `the diameter ofthe tun'gstenfilamenty 42.',is'- 0.020 inch... a satisfactorydcposit can ybe obtained with. acurrent oi 131amperes and gold 'droplet'sitot'alling -1/'2 dwt. in weight in from 3 tof'15`seconds. For a strip approximately 5 X 5.5 mm2, the .resistance of the Ydeposited'gold will generally liein the range of from50-to75 ohms.

After the strip 26 has vbeen deposited lasfdescribed above; `the eleetrodesassernbly is removed fromthe bell jarv36, the strip maskis removed and the electrode masks are replaced." Theelectrode. assembly isi then 'placed in 'the fbell f jar 36 and' goldl is vagain evaporatedv onxthe electrode areas `28 and '29 rin Fig; 2 under' the isamefcondi tionsand'in vthe ysame .manner'a's 'described above. It :has been found' that. this ksecond electrode dcposit lowersthe'resistance of the `unit and improves the yield of quietrand! `sensitive units.

Afterfthe gold' electrode area has vbeen lproperly deposited, the electrode` assembly is removed from the bellja'r36 and the. electrode masks are taken off and replaced by the strip masks` preparatory tothezdeposition ofthe gold blackening layer 2l. As vdescribed' in'y the aforementioned copending application, vSerial No.1656,64'0', the electrode assembly With `:thel metal strip r masks f in place is placed under thebell .jar 36 which is 'then evacuated by'means of the pumpf'38 to a pressure of about'g05 mmfof mercurytorless.- Azdry', inert or inactive .gassuchuas nitrogen; helium',v argon or neon is then ,admitted'into' thebell'jar until the pressure'lies Within'theirange from 2 to :12 mm; of .mercury` 'Current is then passed' through the tungsten filament `i2 until large quantities'of black ororange-black vaporor fogbegin to come from the golddropsron the lament, thusfcrming the -gold black layer 2Tv on the metalA strip .26. Afterremoval from the belljar the electrode 'assemblyis then :preferably a'g'ed'by dissipating approximately' 50 .milliwatts Yof electrical` energy. lin thermetal" strip 26 "for: a Vperiod .of from. 2.1'to 3 hours.

iIthas been observediithat therfresistance of theY electrode'V assembly `drops substantially .after the second electrode depositis putonand-.that there; is alfurther drop after the Vlatterzhasibeen removed from-.the'bell jar 36. Theaging process referred tol above also produces a further reduction :in resistance. sirable ftoterminate the .evaporation of gold during .theaformation of. the metal strip v26. wherrthe electrical :resistance measured between'. the rods i8 and I9 while the electrode assemblyis inthe bellljar 36y is approximately twicethelnal resistance desired for the bolometer. .Whenthe plasticiilm .'25,vthe metal strip` 2B andthe' gold black layer 2'! have been formed on the electrode assembly and after the electrode assembly has/.been aged as" described above, it may be sealed WithinA thefcylindricalenvelopay. This may be accomplished by slidingthe'lower base member l2 withinthe cylindricalenvelope 30 and sealing oif'the bottom joint by'loW temperature-solder as at 3|, or in `any other suitable manner. The completed assembly is then placed in an. open-sided oven and baked for a period of from 2 to 3 hours-at a-'temp'erature from 140 to 145i 'F'. While-under the vacuum provided by a rctarypump.. At theend-oftheik'ing operation,

For these reasons, it is 'de s; .dry. .inactive vorzinert gas such as nitrogen. argon for neon, .for example, isv admitted'rto.y the bolometer to an 'excess pressure and. is lthen. pumped down Vto al suitable pressure Whichfmay li'ezwithin the frange of 2 mm.ito 760 mm. of `mercury. The bolometer is then sealed oilin the conventional manner to prevent leakageof` atmospheric air into theinterior thereof.

:In order to insure a satisfactory sensitivity andA frequency characteristic, it isessential-Lthat the proper spacing be maintained vbetween the raised-portion 52 ofthe cooling plate 5l and the metal strip 26. .It can be shown that .if sinusoidally varying infra-red radiation impinges upon the metal strip 26, the amplitude of the temperature Wave transmitted in thevmediumnin a direction perpendicular to the Vface of themetal strip 26 will be reduced to 4.32% of its initial valuein adistance of 1/2 wave length; sovth'at events occurring at greater distances from Vthe surface ofthe. strip 26 are of no practicalinterest.. `If .the cooling plate 5Iv is spacedfrom the strip by only 0.2i a thermal wavemusttravel a distance of GAA in .going from the strip-26 to the raised portion 'of the plate 5l and back. Aand theover-all attenuation will .reduce the amplitudeof the vreflected Wave to approximately 0.081 of the original temperature varaition of Vthe strip. Hence, if' the effect of the. gas and platel on the strip- 26 is todiifer markedly from that of the gas alone, the raised portion 52 'of the plate 5|.must be maintained within' .2i 4of the strip. As a practical matter it can be shown that in order to obtain'. aV frequency characteristic which is reasonably independent 'of' frequency.

the spacinghould beof the order of 0.1 to 0.3 mm. Glass fiberor fibers 6Irnay be'us'ed to maintain this vspacing if desired.

' In the embodiment illustrated in Figs. 5-8'inclusiveof the drawings, apair of crescent-'shaped electrodes 52 and 53 are secured to the'roclsl I8 and I9 and a plastic lm 54 is .formed on them as shown 'in Fig.' 6. A metal strip 55` having a layer of black gold 56 thereon, which may 'be applied to the plastic lm in? thesame manner as strip "26 and layer 21 of Fig. 1, extends between the electrodes 52 and '53. In' this embodiment.'- a cooling plate 51 is imbedded in a' disk :58 of elec tricalV insulating material such yas a polystyrene,

' forexamplef-Thedisk V5'6 is provided with opposite U-sh'ap'ed'slots 59 andV '60 to accommodate the rods I8 and'l9 and-is cemented Vto' the lower surface of the'electrodes' 52 and 53 such that the cooling plate '51 lies directly below the fmetal strip`55. If' desired, one or more vglass fibers 5| Ymaybe disposed` 'betweenV the plastic lm '5l and the `cooling plate 5l 'as shown in Fig'.` 1;'.in ordertol support the 'film 54.

If' desired, heavymetalelectrodes '62- and? 63 `may be provided having a centrally locatedfrecess 54 formed inthe upper-face thereof, as shown in Fig'. 9. In this embodiment,v the central portions of 'thefeleetrodes 52 and '63 forming the bottom ofthe recess '64 'serve 'as the cooling plate; 'and the depthaofthe recess-601smadeequal tothe desired spacing betWeen'the plastic iilmrfkahd the vcooling plate.

Bolom'eters of "the type' describedaboveare especially useful `for detecting radiant energy in the infra-red region, the intensity of which'varies periodically at a relatively low' frequency; say 50 cycles per second or less. By controlling the gas content, the gas pressure and the spacing between the cooling plate'and the conducting:metalzstrip.l

y a relatively f widerzvariety. .of tfrequency Lresponse characteristics may be obtained. For'optimum operation at a xed frequency, with the maximum safe current flowing in the strip, a bolometer should preferably be designed so that its frequency characteristic drops by approximately 3 db at the operating frequency.

It will be apparent to those skilled in the art that the above-described bolometer apparatus and processes by which it is made may be modified'in a number of respects Within the scope of the" invention. For example, several alternate methods of depositing the gold on the electrodes and forming the metal strip may be used. If' desired, the metal strip may be deposited rst and a layer of gold may be deposited over it above the electrodes, or the second deposit of gold on the electrodes in the preferred method described herein may be omitted] Instead of the specific iilm solution described above, other cellulosic materials capable of iorming thin films maybe used. Agfa Ansco No. 353- 330- nitrate lm dissolved in amyl acetate or any other suitable solvent may be used. The emulsion can be left on the lm if desired, and ltered outof the solution. A mixture of one part Duco cement and three parts Duco thinner No. 3614 vGr also makes a usable nlm solution. Other suitable cellulosic materials will suggest themselves to those skilled in the art.

From the foregoing, it will be understood that the invention provides new and improved bolometers which have good sensitivity and low frequency response. By utilizing a cooling plate located a suitable distance behind the metal heat responsive strip, an inert gas at a pressure substantially higher than is used in hydrogen filled bolometers may be employed. Bolometers constructed in this fashion are much less influenced by leakage of atmospheric air into the envelope or by outgassing from the elements contained within the envelope.

While several representative embodiments have been described herein, invention is not to be limited in any way thereby but is susceptible of numerous changes in form and detail within the scope of the appended claims,

What I claim is:

1. In a bolometer, the combination of a sealed, metal, tubular envelope containing a gas and havingr a window capable of transmitting radiant energy in the infra-red region therethrough, a pair of insulating beads mounted in said envelope and carrying a pair of longitudinally extending conducting rods, said conducting rods providing electrical communication between the interior and exterior of said envelope, a pair of flat electrodes rigidly secured to said conducting rods, said electrodes having cooperating spaced-apart portions l lying in* a plane substantially perpendicular to the longitudinal axis of said envelope and forming a ring-like support, a plastic lm carried by said support, a thin, blackened metal strip on said film and electrically connected to said electrodes, and a polished cooling plate supported in close proximity to said metal strip on the side thereof opposite to that from which radiation is received.

2. A bolometer comprising a sealed, metal tubular envelope containing nitrogen at a pressure in the range from about 2 mm. to 760mm. of Inercury and having a Window capable of transmitting radiant energy in the infra-red region` therethrough,-a pairof insulating beads .mounted in said envelope` and carrying a pair oflongitudinally` extending conducting rods; said conductingv rods metal strip on said lm and electrically connected to said electrodes, and a cooling plate positioned on the side of said metal strip opposite that from which radiation is received and having a polished,

raised portion lying beneath and in close proximity to said metal strip.

3. A bolometer comprising a sealed, metal tubular envelope containing nitrogen at a pressure in the range from about 2 mm. to r160 mm. of mercury and having a window capable of transmitting radiant energy in the infra-red region therethrough, a pair of insulating beads mounted in said envelope and carrying a pair of longitudinally extending conducting rods, said conductingl rods providing electrical communication between the interior and exterior of said envelope, a pair of flat electrodes rigidly secured to said conducting rods, y said electrodes having cooperating, spaced-apart portions lying in a plane substantially perpendicular to the longitudinal axis ofsaid envelope and forming a ring-like support, a plastic film carried by said support, a thin, blackened, metal strip on said lm and electrically connected to said electrodes, an insulating plate supported beneath said electrodes, and a polished, metal plate imbedded in said insulating plate, said metal plate lying in close proximity to said metal strip on the side thereof opposite to that from which radiation is received.

4. A bolometer comprising a sealed, metal tubular envelope containing nitrogen at a pres` sure-inthe range from about 2 mm. to 760 mm. of mercury and having a Window capable of transmitting radiant energy in the infra-red region therethrough, a pair of insulating beads mounted in said envelope and carrying a pair of longitudinally extending conducting rods, said conducting rods providing electrical communication between the interior and exterior of said envelope, a pair of iiat, semi-circular electrodes rigidly secured to said conducting rods, said electrodes lying slightly spaced apart in a plane substantially perpendicular to the longitudinal axis of said envelope and forming a disc-like support. means forming adjacent recessed portions in said electrodes between said conducting rods, a plastic film carried by said electrodes and lying over the adjacent recessed portions thereof, and a thin, blackened metal strip on said film and electrically connected to said electrodes, the bottom walls of the adjacent recessed portions in said electrodes cooperating to form a cooling plate in close proximity to said metal strip on the side thereof opposite to that from which radiation is received.

CHARLES B. AIKEN.

' REFERENCES CITED The followingreferences are ofrecord in the ille of this 'patentz' A UNITED STATES PATENTS 

